An ongoing swarm of earthquakes under Mount St. Helens doesn’t mean the volcano is likely to erupt soon, but scientists are studying the tiny temblors to see what they reveal about magma movement deep underground.
The quakes, none bigger than magnitude 1.3, started chattering under the Northwest’s most active volcano on March 14. Since then, the Pacific Northwest Seismic Network at the University of Washington has logged well over 130.
The quakes are triggered as molten rock moves into St. Helens’ magma chamber about five to six miles below the surface, said Seth Moran, scientist-in-charge of the U.S. Geological Survey’s Cascades Volcano Observatory.
“What’s going on at the volcano is nothing that new,” Moran said.
In 2014, the USGS reported that several subtle signs, including small quakes and slight motion of GPS stations on the volcano’s flanks, indicated that the magma chamber was inflating slowly, or recharging.
“We issued a statement then that we think we’re seeing evidence that magma has entered the system,” Moran said. “It’s getting ready for the next eruption, which is years to decades down the road.”
The latest cluster of quakes represents an uptick in that process — but the seismicity isn’t powerful enough or close enough to the surface to indicate that an eruption is imminent, he said. Also, there have been no unusual gas emissions or rapid ground motions that usually precede eruptions.
The volcano went through a prolonged period of recharging after its cataclysmic 1980 eruption. Earthquake swarms much more energetic than the one under way now occurred throughout the 1990s, but the volcano didn’t start erupting again until 2004.
“What’s happening now is well within the realm of what we understand to be normal behavior for Mount St. Helens when it’s in the recharging phase,” Moran said.
But for scientists, the swarm presents an opportunity to dig more deeply into the physical processes that produce the shaking, he added.
As molten rock moves into the underground chamber, it exerts pressure on the surrounding rocks, which can lead to fractures and earthquakes, said Wendy Bohrson, a geologist at Central Washington University who studies magma chemistry.
“It’s a bit like trying to force more volume into any fixed container,” Bohrson said. “The rock that sits around the magma chamber might crack a little bit.”
At Mount St. Helens, some of the recent, tiny earthquakes appear so similar to each other that the scientists call them “repeaters.” The same faults appear to be fracturing repeatedly in the same way, Moran explained.
“There’s some sort of regular process that is producing stress, causing this one fault to rupture, then the stress continues to build and the fault ruptures again,” he said. “It requires some steady-state process.”
By teasing out the details in the seismic signals, the scientists hope to figure out more about that process — and whether it might shed light on how volcanoes recharge and build toward their next eruptions.
“The goal is to see if there’s information we can use,” Moran said.